1. Field of the Invention
This invention relates generally to a two-component mixing type coating method which is suitable for coating car bodies, machine components, for example. The method uses a coating such as a urethane coating consisting of a mixture of two components, i.e., a main component and a hardener.
2. Discussion of the Background
The coating of car bodies, machine components, and the like, has been conducted in the past by the use of a two-component type coating. For this type of coating work, a coating supply method has generally been employed which supplies a main component and a hardener from their inividual supply sources to a coating spray gun by operating respective pumps. The main component and the hardener run together at a junction at an intermediate portion of the supply paths, mixes them together by a mixer and supplies the mixture to the spray gun. In most cases, a check valve or valves are disposed in one, or both, of the component flow paths upstream from the junction. During the supply of the main component and the hardener, particularly immediately after stopping the spraying operation of the spray gun, the main component is likely to enter the flow path of the hardener flowing from the junction or vice versa due to the difference in the supply pressures between the main component and the hardener. The check valves are disposed in the supply paths in order to prevent such a backflow.
However, when the paint spray gun is repeatedly operated and stopped and the backflowing main component (or the hardener) enters the check valve many times, the main component and hardener are mixed together and cured, and this cured product limits the opening and closing operation of the check valve. As a result, the check valve becomes fixed in a state where it is somewhat open and cannot be closed fully. Therefore the backflow cannot be prevented effectively.
Carefully noting the fact that the backflow described above occurs most often immediately after stopping the spraying operation of the coating spray gun, the inventors of the present invention have proposed placing stop valves disposed between the junction and the check valve in addition to the construction of the conventional two-component mixing type coating apparatus described above. See applicants copending U.S. Pat. No. 4,703,894 entitled "Two-Component Mixing Type Coating Apparatus" and filed on the same date as the present application and incorporated herein by reference. In this type of apparatus the backflow can be checked by the check valves during the spraying in the same way as in the prior art apparatus. Additionally, the stop valves are closed to close the supply paths when the gun stops spraying in order to reliably check the backflow. From the aspect of safety, an air pressure piping arrangement is generaly disposed between the spray gun and each stop valve as the connecting system between the coating spray gun and the stop valve. When the air pressure increases or decreases due to the stopping of the spraying operation of the gun, the pressure change is fed back so as to close the stop valve.
Although the system described above effectively prevents backflow, there is a considerably great time lag from the stopping of the spraying operation of the spray gun to the closing of the stop valve. Accordingly, when backflow occurs, the backflowing main component or hardener can enter the path near the check valve through the stop valve before the stop valve is closed. This adversely influences the opening and closing operation of the check valve.